Guide:Guaranteeing a 99 Roll Against the Netherbrain

When first ⁠Confronting the Elder Brain in the ⁠Morphic Pool, the speaking character must make several ability checks to attempt to subdue the brain with the Netherstones. These checks all have a DC of 20+ and are made more difficult by the fact that they are not associated with any skill and therefore cannot benefit from a character's skill proficiencies or expertise. These checks are often seen as pure tests of luck, especially the final check, which has a DC of 99 and which many people believe can only be passed with a natural 20. However, with enough setup, it is possible for a dedicated player to guarantee success, even in Honour mode.

Setup

The crux of this strategy revolves around having a  Wild Magic Sorcerer give as many stacks of  Bend Luck: Ability Check Bonus as possible to the speaking character in the Netherbrain cutscene (the sorcerer cannot be the speaking character). While technically, only six levels in Wild Magic Sorcerer are required to pull off this strategy, it requires a huge amount of  Sorcery Points, so to avoid excessive tedium or resource use, you may want to take more levels in sorcerer.

Other buffs

It is helpful to have other characters in the party with features that buff ability checks to speed up the buffing process, so here is a list of all the non-conflicting ability check buffs that I could find:

•  Guidance (requires concentration)
•  Bolstering Magic: Boon
•  Cosmic Omen: Ability Check (requires concentration)
•  Bardic Inspiration (class action)
•  Dark One's Own Luck (requires the dialogue character to use themselves)

However, seeing as the Netherbrain cutscene is past the point of no return – and  Withers will not be able to respec party members afterwards – the final section of the game could become excessively difficult if you abandon more combat-focused builds to get these features. You may choose to use some or none of these features and still guarantee success.

Now, this strategy can work with any of the six ability scores, but to achieve the highest result – and as said before, reduce tedium –  Strength is far and away the best option to build around. All you need is to have the dialogue character drink an  Elixir of Cloud Giant Strength to give them a +8 to the final check. You should also have them carry the  Resonance Stone for  Advantage on Strength checks. Optionally, you can also have them wear  Dwarven Splintmail or one of the other armours with  Body Aid to get an extra +1 to Strength checks, but this is likely unnecessary.

Maximising Sorcery Points

In order to maximise the number of  Sorcery Points on your Wild Magic Sorcerer, it is also helpful to have items that restore spell slots or grant new ones, which can then be converted into  Sorcery Points. Here are all the ones I could find:

•  Pearl of Power Amulet
•  Spellcrux Amulet
•  Shield of Devotion
•  Shield of the Undevout
•  Spell Savant Amulet

Finally, bring with you as many  Potions of Angelic Reprieve as you can, as each one can restore up to 10 spell slot levels for your sorcerer. Note that  Potions of Angelic Slumber will not work for this strategy, as consuming one will reset a sorcerer's  Sorcery Points, just as when they take a long rest. The fewer buffs you have available, the more potions you can use to compensate.

If your sorcerer character is a partial-illithid, you can use  Freecast to get 5 more  Sorcery Points. If you are still short on  Sorcery Points, then – as noted in the Bugs section of the Freecast page – you can continually reset Freecast by repeatedly having another character apply a condition (such as  Guidance or  Aura of Protection) to your sorcerer. With this, you can get effectively unlimited  Sorcery Points, with enough time, and can guarantee a 99+ ability check.

Calculating minimum roll

If you'd like to calculate what buffs are needed for success before embarking to the Morphic Pool, you can use this AnyDice script to determine how close you are to rolling at minimum 99. Just replace "2d10", "1d6", and "32d4" with the number of dice you'll have buffing your own roll (use 0 in front of a dice size if you have none). Click "Calculate" and then "At Least" to see the table that shows your odds of rolling at least 99. By experimenting with this, you can see that with only d4's buffing your roll, you would need at least 45d4's to guarantee success. Remember that each stack of  Bend Luck: Ability Check Bonus costs  2 Sorcery Points, so if your only other buff was  Guidance, then you would need 44 stacks of Bend Luck, and therefore  88 Sorcery Points to guarantee a roll of 99+.

Method

Once you've reached the Morphic Pool, arrange your characters near the cutscene area. Your dialogue character must be close enough to get to the cutscene trigger point within a single turn. The cutscene trigger area is the last circular patch of normal stone before the veiny and fleshy ground in front of the Morphic Pool.

Ensure that your sorcerer has converted all of their spell slots into  Sorcery Points, then use any 10-turn buffs like  Bolstering Magic: Boon and  Cosmic Omen: Ability Check before entering turn-based mode.

Have your sorcerer repeatedly use  Bend Luck: Ability Check Bonus on your dialogue character until the sorcerer runs out of  Sorcery Points. If your dialogue character has Bardic Inspiration, ensure that the  Bardic Roll passive isn't set to auto-trigger.

Finally, move your dialogue character into the cutscene. The first three rolls shouldn't be any problem, even if you use abilities other than Strength. On the final ability check, just make sure that you activate any remaining buffs like  Dark One's Own Luck and  Bardic Inspiration. You can technically still fail the ability check if you roll two natural 1's, so you may still want to have some inspiration points, just in case.

Conclusion

Ultimately, this is a lot of effort for a 20% HP reduction against the Netherbrain, and using less combat optimised builds – like a Wild Magic Sorcerer – probably causes a greater increase in difficulty than the loss in health offsets. At least you can say you did it.

This method also works (and much more quickly) for any other ability check in the game, so if you need to guarantee a  DC 30 Sleight of Hand check to pickpocket something important, maybe consider bringing along a Wild Magic Sorcerer.